Gelation behavior of cellulose in NaOH/urea aqueous system via cross-linking

Sol–gel transition of cellulose solution in NaOH/urea aqueous solution with the addition of epichlorohydrin (ECH) was investigated by rheological means. The gelation was controlled by a synergy of chemical and physical cross-linking processes, namely, the etherification reaction between cellulose and ECH as well as the self-association and entanglement of cellulose chains via hydrogen bonding re-construction in NaOH/urea. The results revealed that the cross-linker concentration, cellulose concentration and temperature played important roles in the gelation behavior. The gel time decreased with increasing either ECH or cellulose concentration, and the gel temperature dropped from 38 to 28 °C with an increase of cellulose concentration from 4 to 6 wt%, i. e. easier gelation was reached with higher cross-linker concentration, cellulose concentration or temperature, since higher cross-linker or cellulose concentration led to more network junctions via chemical or physical cross-linking, while higher temperature was favorable to both the etherification reaction and re-construction of cellulose hydrogen bonds. The compressive modulus of cellulose/ECH hydrogels was improved a lot by increasing either cellulose or ECH concentration, indicating the chemical cross-linking obviously improved the mechanical property, on the other hand, the swelling property could be tunable by changing the gelation parameter. This work supplied useful information to the control and optimization of the structure and properties of cellulose based hydrogels.